For some time it has been known that highly corrosion and wear resistant films may be deposited on metallic surfaces by a number of PVD or CVD processes. Common applications include aerospace, nuclear and tool materials. Indeed the gold colored, premium cost, tools available to industry and consumers generally have a thin surface layer of titanium nitride (TiN) applied onto them by PVD techniques. TiN has demonstrably increased the lifetime of drilling and cutting tools several times when compared to similar non-coated tools.
Various substances, such as TiN, TiC, ZrN, HfN, TaN, TiAlN, TiO.sub.2, ZrO.sub.2, HfO.sub.2, Ta.sub.2 O.sub.5, etc. have been synthesized by vapor deposition processes as coating materials. These materials exhibit a number of special properties.
Tools or larger parts are exclusively coated by a batchwise PVD technique. The parts are placed in a reactor on a table-like fixture and then coated. In some cases the parts are placed in a Christmas tree-like fixture mounted on a rotatable table. Loading and unloading of parts is thus labor intensive. Accordingly, table-type fixturing devices are preferred to be used in the coating of relatively expensive parts. The cost for table-fixture coating these parts might appear quite high in absolute terms but is not too significant in relation to the total cost of the parts. For example, coating costs of 0.50 cents per piece that normally sell for $10 may be a tolerable expense. However, the same coating cost cannot be justified for parts of very low cost.
Accordingly, vapor deposition technology has been selectively applied to microelectronics and a variety of relatively higher priced consumer goods. However, the technology has not been applied in situations where the coating of large quantities of inexpensive objects is required. One such area is the production of coins, coin blanks, discs, medals, tokens, ball bearings, etc. For example, the surface of coins must have an attractive appearance that should be retained for a long time while the coins are in circulation. Thus it is required that the coating exhibit high corrosion and wear resistances.
Recently, there has been a decided push towards the development and usage of gold colored coinage. See, for example, Canadian Patent No. 1,203,723 and note the recent introduction of the golden color Canadian dollar coin. The British pound, Australian dollar, French 10 franc and Spanish 100 peseta have also been recently issued in a variety of bronzes.
Experiments dealing with titanium nitride deposition on various coins using a reactive plasma spraying system developed by Multi-Arc Vacuum Systems Inc. of St. Paul, Minn. have been conducted by applicant. It has been determined that coins of a very attractive golden appearance, which could be retained for at least 20 years in circulation, may be produced by depositing approximately a one micron thick TiN layer on the coins.
Briefly, the PVD system employed for the experiments included a grounded closed chamber (reactor) comprising a pair of negatively charged power supplies (-17.5 volts) communicating with a corresponding pair of metal evaporators (cathodes) disposed within the reactor. When energized, the evaporators (in this case titanium evaporators) emitted a plasma stream of positively charged titanium ions. A fixture table, negatively charged by an external bias supply (-150 to -400 volts) and used for supporting the materials to be coated, was also disposed within the reactor and positioned to be bombarded by the titanium ions. A vacuum pump and means for supplying a reactive gas (such as nitrogen) to the reactor completed the system.
In operation, the titanium ions are attracted to and bond to the table and objects thereon. By introducing the reactive gas into the chamber, the nitrogen and titanium combine on the substrate forming TiN--a hard, corrosion resistant gold colored coating.
However, production problems associated with handling and fixturing the coins were encountered. The slow production rate would be a severe constraint in making the process economically feasible when using the table fixture. The coins to be coated had be placed on the table in a single layer. During deposition only the upper side of coins (exposed to the Ti cathode) were coated. Therefore following the coating of the exposed coin side, the reactor had to be opened, the coins cleaned and then put on the table with the other side up. The coating process then had to be repeated.
Thus it could be seen that the process was slow, labor intensive and, as a consequence, not economically feasible for use with small objects. The system, as it stood, precluded the economic coating of small parts.